Make upgrade logics public

[larry0x]

Currently cw721_base::upgrades is not exported, but it should be. It is needed by custom NFT contracts that extends cw721-base to implement their custom migration logics.

Let's consider a contract called sg721-base that extends cw721-base. Also assume:

• sg721-base 0.25 uses cw721-base 0.16
• sg721-base 0.26 uses cw721-base 0.17

How should sg721-base implement its 0.25 => 0.26 migration? Imo, the cleanest way is this:

const CONTRACT_NAME: &str = "crates.io:sg721-base";
const FROM_VERSION: &str = "0.25.0";
const TO_VERSION: &str = "0.26.0"

pub fn migrate(deps: DepsMut, env: Env, msg: MigrateMsg) -> StdResult<Response> {
    // check the correct contract is being upgraded,
    // and it's being migrated from the correct version
    cw2::assert_contract_version(deps.as_ref(), CONTRACT_NAME, FROM_VERSION)?;

    // update contract version info
    cw2::set_contract_version(deps, CONTRACT_NAME, TO_VERSION)?;

    // run the base contract's migration logics
    cw721_base::upgrades::v0_17::migrate(deps)?;

    // other custom migration logics specific to sg721...
}

This is somewhat similar to how inheritance works in Python:

Class Cw721Base:
    def migrate(self, deps):
        # base contract migration logics...

Class Sg721Base(Cw721Base):
    def migrate(self, deps):
        # run base contract migration logics:
        __super__().migrate(deps)

        # other custom migration logics specific to sg721...

See how the child contract needs to invoke cw721_base::upgrades::v0_17::migrate similar to how the python class invokes its parent's method by __super__.

Similarly, if a contract extends sg721-base, then it should also invoke sg721_base::upgrades::v0_26::migrate in its custom migration method.